Production of niobium



Nov. 17, 1964 J. N. ROBINSON PRODUCTION 0F NIoBIuM Filed Jan. 22, 1962 F US E D SALT ZINC In ven tor Mase/9h /1/ Rob/5072 .d 2/2/ 2/ 2/ /V 7% f 6 T 2/ M\ MH\ Altorne y United States Patent O 3,157,493 PRDUCTIUN F NEEETJM Joseph Newton Robinson, Trail, British olumhia, Canada, assigner to The Consolidated Mining and Smeiting Company of Canada Limited, Montreal, Quebec, Canada, a corporation of Canada Filed dan. 22, 1962, Ser. No.. 367,764 8 Claims. (6l. 7S-84.5)

This invention relates to a method for the production of niobium metal from niobium pentachloride. It is particularly directed to providing a method for the production of niobium by a sequence of chemical reactions in which niobium pentachloride and an alkali metal uoride are reacted to form alkali metal niobium fluoride which, in turn, is reacted with zinc to form a niobium-Zinc reduction product.

Processes for the production of niobuim are known in which niobium metal is produced from potassium niobium fluoride by reaction with sodium or by electrolysis. rl`he prior art processes have the disadvantages that they involve a relatively complicated and costly sequence of operations, and, in the case of sodium, require the use of a reactant which is costly, diicult to handle and potentially hazardous in handling, transporting and storing.

I have found that the diiculties heretofore encountered in the production of niobium can be overcome by reacting niobium pentachloride with an alkali metal uoride, preferably sodium or potassium fluoride in a fused bath of alkali metal chlorides, to form an alkali metal niobium iluoride and reacting the alkali metal niobium uoride With Zinc to produce a niobium-Zinc reduction product from which the niobium and zinc can be separated and separately recovered.

The starting material for the method which forms the subject matter of this invention is niobium pentachloride. Processes for the production of niobium pentachloride are known and form no part of the present invention. Illustratively, a satisfactory procedure for forming niobium pentachloride is to react a porous bed of ferro-niobium pieces with chlorine gas in a reaction zone to form a gas stream which contains ferrie chloride and niobium pentachloride as volatile chlorides with concurrent formation of ferrous chloride. The stream of volatile chlorides is passed upwardly through a porous bed of sodium chloride pieces whichis positioned above the reaction zone and maintained at a temperature above the vapourization temperature of niobium pentachloride but below the melting temperature of ferrous chloride. Ferrous chloride is separated from the stream of volatile chlorides as it passes through the sodium chloride bed. Niobium pentachloride is separated and recovered from the stream ogvolatile chlorides after separation of the ferrous chlori e.

The method of this invention comprises, in general, the steps of feeding niobium pentachloride into a fused mixture of sodium chloride, potassium chloride and alkali metal fluoride in a rreaction zone in amount sufcient to combine with the niobium content of the niobium pentav chloride as potassium-niobium fluoride double salt, maintaining a non-oxidizing atmosphere in the reaction zone, providing zinc in said reaction zone for reaction with the potassum-niobium iluoride double salt in amountsuihcient to form a niobium-zinc reduction product, separating niobium-zinc reduction product from the fused saltV bath, and separating and separately recovering niobium and zinc from the niobium-zinc reduction product.-`

ln the operation of the present method, niobium pentachloride, NbCl5, dissolves in the fused salt bath to form potassium niobium fluoride, KZNbFq. It is found that the niobium-zinc reduction product forms in the zinc layer only to a limited extent. Most of the niobium-zinc reduction product forms as a deposit, either on the tixture 14, shown in FIGURE l and described in detail hereinafter, or on the walls of the container above the level of the molten Zinc bath, as illustrated in FIGURES 2 and 3, which indicates that the reduction actually occurs in the fused salt bath. Thus, it is believed that the reduction of the` potassium niobium fluoride and the formation of the niobium-zinc reduction product are caused by the zinc dissolved in the fused salt bath. The reducing reaction can be explained as proceeding in the fused salt bath which, in the upper part, where the niobium pentachloride is fed into the bath, is rich in potassium niobium fluoride and low in zinc, and which, in the lower part adjacent to the zinc layer, is low in potassium niobium fluoride and rich in dissolved zinc.

It is found that the best results are obtained in the operation of the method when an adequate supply of zinc is dissolved in the fused salt bath. The zinc supply in the fused salt bath can be increased by increasing the temperature of the bath or, preferably, by providing a small how of inert gas, such as argon, through the zinc layer into the salt bath. The inert gas serves to agitate the zinc and, possibly, carries zinc vapour' into the salt bath. it also serves to agitato the Salt bath. It is found that if the flow of inert gas is too strong, niobium zinc reduction product tends to deposit around the niobium pentachloride tube and thus closes the outlet end. The ilow of inert gas should be sufficient to agitate the pool of zinc and the salt bath gently to ensure distribution of the reactants throughout the bath.

The apparatus for carrying out the method for the production of niobium of this invention comprises a container enclosing a reaction zone, an internal lining for the container resistant to attack by fused alkali metal chlorides and duorides, inlet means for feeding niobium pentachloride into said container, inlet means for feeding an inert gas into said container, means for providing a supply of molten zinc in said container exposed to the fused salt bath, and means in said container exposed to the fused salt bath for collecting niobium-zinc reduction product.

An understanding of the method of this invention can be obtained from the following description, reference being made to the accompanying drawing in which:

FGURE l is an elevation in section of an apparatus suitable for the production of niobium by the method of this invention;

FIGURE 2 is an elevation in section of the apparatus illustrated in FIGURE 1 in which the zinc metal is positioned as a molten bath below the fused salt bath; and

FIGURE 3 is an elevation insection of a further modiiication of the apparatus illustrated irl-FIGURE 1.

Likereference characters refer to like parts through-l out the description of the invention and the drawing.

Referring to FGURE 1, the numeral il@ indicates a container of suitable shape, for example, rectangular or circular. it preferably is formed of a stainless steel outer shell and is lined with a lining il, such as graphite, which removable cover 16.

A graphite tube 13 extends through an opening in the cover 16 downwardly into the fused salt bath 12 and terminates above the bottom ofthe container a distance sufficient for the growth of a niobium-zinc alloy reduction product agglomerate 30 to a desired size without contact with the lining 11. Zinc is fed into the fused salt bath 12 through the tube 13. Conveniently, a zinc rod '7 can be fed into theupper end of the` tube 13 and advanced towards the `lower end. As the temperature of the reaction zone is substantially above the melting point of zinc, the lower part of the rod melts, as illustrated by thedroplet indicated by the numeral 7a, below the top of the tube and falls through the lower part of the tube into the fused salt bath at the lower end of the tube.

A` fixture 14l is secured to and encloses the lower end bath, preferablyv centrally. It is formed of a material such as graphite, which is resistant to corrosive attack by the fused salt bath, with' openings 14n for the circulation of molten zinc andl fused salt into and through it. Itxprovides a core on which niobium-zinc reduction productii()y collects and grows as the reaction between the potassium niobium fluoride and the zinc proceeds. When ithas grown to a desired size, the graphite tube and fixture are-removed together with the niobium-zinc agglomerate attached to thev fixture, another graphite tube and fixture are inserted', and the reaction is continued;

An inlet pipe 17 is provided in the upper part of the container for, the admission of an inert gas, such as argon, which serves to maintain an inert atmosphere in the container.

An inlet pipe 8, connected at its outer end to a source of niobium pentachloride, extends from the top of the container to a point adjacent to the bottom. Niobium pentachloride isrfed through this pipe into thel fused salt bath 12'.

.The pipes 8 and 17 preferably are formed of graphite, toV ensure resistance to corrosion.

FIGURE 2 illustrates an alternative arrangement of the container and ancillary parts. The shell 20, the liningg21, the removable cover 22, the niobium pentachloride and inert gas inlety pipes 23 and V243-, respectively, are the same as in FIGUREfl described above. A pool of molten zinc metal 25 is provided in the bottom of the Y' container.V A fused salt bath 28 is positioned above the pool of molten zinc. In this modification, a deposit of niobiumazinc reduction product 29 is formed and grows on the graphite liner in the area covered by the fused salt bath. The lower open end of the pipe 24 terminates in the bath of molten zinc. v

FIGURE 3 illustrates a modification of the' apparatus illustrated inFIGURE 2 in which a removable liner 26 y is provided in the' container 20. This liner also is formed of materiah for example, graphite, which is inert to y.c orrosiveattaclt by the fused salt bath. It extends from the bottom of thecontainertoa short distance below the K top. A'This liner Ztfprovides a surface on which the niobium-zinc reduction product collects, as indicated by {thefnumeral 29a,` as the reaction proceeds between the zinc and the potassiumniobium fiuoride.

When the niobium-zinc deposit. has grown to adesired size, the linerZY and niobium-zine.'deposited thereon are removed from the container. Y.

Containers -20 can be heated to and maintainedfat VVthe desired reaction temperature, ,which is above the melting temperature of the fused salt bath but below. the vapourization point of zinc, preferably within the range of from about 700 C. to about 880 C., by the use of conventional heating means, such as electric resistance coils, not shown, or it can be heated to reaction temperature in a heating furnace, or by circulating the fused salt bath and maintaining it at reaction temperature outside the container.

Zinc can be separated from the agglomerates 29, 30

and 29a by vacuum distillation, leaving a niobium prodf uct'which is substantially free from zinc and other irnpurities.

The reactions involved in the operation of the method can be expressed by the following equations:

The fused salt bath can be prepared. with 1.0 part potassium chloride, 0.262' part sodium chloride, 0.393- part sodium fluoride. When saturated. with niobium pentachloride, this fused salt mixture contained, by weight,

40% medium chloride, 40% potassium chlorideand 20% potassium niobium fluoride.

Theoretically, a total of 1.317 gramsvof zinc is required. per gram of niobium pentachloride, 0.592 gram of zinc being required for the reduction of potassium niobium fluoride and 0.725 gram being required for niobium-zinc' crystallization as NbZn3 For optimum results in the operation of the process, zinc should be provided in excess of the theoretical requirement for formationof theV compound NbZn3. If the amount of zinc present during the process is not in excess of the amount theoretically required to form NbZn3, the niobium-zinc reductionV product is deposited as very fine crystals. This fine.r crystal size causes difiiculty in the subseqeunt recovery of the niobium after removal of the zinc. Preferably, therefore, more zinc is provided thany is required for formation of the lcompound NbZn3, which is deposited in coarsely crystalline form in the presence of excessA zinc.

I have found that very satisfactory results are obtained in the Voperation of the process at a temperaturewithin the range o f from about 700 C.'tovabout 880 C., preferably about 750 C. The melting point of a'lzly molar sodium chlorideapotassium chloride fused salt bath is about 658 C. and the maximum temperature of-they bath is limited to 905 C., the boiling pointof zinc.

In the operationof the process in the container illus.- i Y I trated inFIGURE 1, niobium pentachloridevapour isV bubbled into the fusedsalt bath through inlet 8. When therfused salt bath is almost saturated withfniobium pentachloride, zinc metal, foriexample, in the form of a. 1 i

forms on the fixture 14. When theV depositjgrows to a desired; size, it is Vwithdrawn from the `container .and a,

new vrod and collection `fixture '14 are inserted. V1\Iiobium'- i zinc alloy deposits can be formed in ,andl extracted from.V

the fused salt bath until the accumulation of zinc chloride `in the'molten bath becomes excessive.

In the Voperation of the method in the container illusf I i trated in FIGURES 2 and 3,a pool of molten zine25 is maintained at thebottom of the container and the 'Y fused salt bath is positioned above the zinc.v Niobium` pentachloride isbubbled into the fused salt *bath` andan" inert gas, such as argon, is'bubbled slowly into the zinc.

This intermetallic reduction product deposits andVAK TheVniobium-zinc reduction product deposits from the fused salt bath onto the wall, as illustrated in FIGURE 2 and on the removable liner 26, as illustrated in FIGURE 3. In the operation of the modification of the method illustrated in FIGURE 3, at the end of a reduction cycle, which is based on a predetermined amount of fused salt bath, zinc and niobium pentachloride, the following sequence of operations is conducted as quickly as possible:

(a) The niobium pentachloride and inert gas inlet pipes in the top of the container are removed.

(b) The removable liner 26 is lifted from the container and the niobium-zinc reduction product is removed, for example, with a stainless steel plunger.

(c) The fused salt charge is removed by displacing the salt into a. holding vessel, not shown.

(d) Additional zinc is charged into the bottom of the container, fresh fused salt mixture is charged into the container, the removable liner and top assemblies are replaced and the reduction cycle is repeated.

An important function of the inert gas is to circulate the fused bath and assist in saturating it with zinc which promotes the formation of the niobium-zinc reduction product.

The niobium-zinc reduction product can be treated by any one of several methods for the separation and separate recovery of the respective metals. For example, the alloy can be leached with dilute sulphuric acid, about 10% normal, to separate excess zinc. The resulting alloy crystals can then be vacuum distilled at from about 900 C. to about 2000 C. to remove the residual zinc by distillation. The niobium residue can then be sintered in a vacuum at from about 1700 C. to about 2000 C. to give the product stability in air. The resulting product is a niobium sponge which can be converted to a powder by mechanical methods. The vacuum distillation and sintering can be conducted as a single operation, if desired, in a Vacuum resistance furnace.

The following examples illustrate the results which can be obtained in the operation of the method of this invention. Example A illustrates the operation of the method in the container illustrated in FIGURE 1 and identified as the Alloy Agglomerate Method. Example B illustrates the operation of the method in the container illustrated in FIGURE 3 and is identied as the Graphite Liner Method. All the percentages are by Weight.

l Added to hath at end of run to react With KiNbFy remaining in bath after NbCl5 addition was stopped.

Total NBC15 reacted 2,258 grams=767 grams niobium. Niobium in 1,837 grams alloy at 35% Nb=642 grams. Percent niobium recovery in alloy: 83.7%.

1 N iohium-zinc alloy recovered from zinc bath. Total NbCl5 reacted=4(1,410)=5,640 grams: 1,920

grams Nb. Niobium. in 4,657 grams NbZn3=4,657 .323 :1,500

grams. Niobium recovery==78-3 The method for the production of niobium of this invention possesses a number of important advantages. Zinc is readily available, it is inexpensive and presents no handling problems. Also, tan-taluni usually is associated with niobium in the niobium bearing material from which the pentachloride is produced and, if so, may be, and usually is, present, in the pentachloride. It is found that alkali metal tantalum uoride does not react readily with the zinc to produce a tantalum-zinc reduction product. Thus, it is found -that niobium which is substantially free from tan-talum can be produced by this method from a niobium pentachloride which contains tantalum. Thus, an essential step in the preparation of niobium pentachloride for the conventional direct reduction system is unnecessary when using the process of the present invention. For example, in tests as described above with niobium pentachloride feed containing 2% to 5% tantalum' pentachloride by weight, 'the tantalulm content of the niobium metal produced ranged from a maximum amount of about 0.1% to an amount not detectable by spectrographc methods.

A further important advantage of the method described above over the conventional direct reduction system is that a completely oxygen-free reduction system and chloride feed material, while desirable, are not essential for the production of low oxygen niobium as any oxygen present forms potassium niobium oxyuoride, K2NbOF5, which is not reduced by the Zinc.

it will be understood, of course, that modifications can be made in the preferred embodiments of the method described above without departing from the scope of the invention dened by the appended claims.

What I claim as new and desire to protect by Letters Patent of the United States is:

1. In a method for the production of niobium from niobium pentachloride in which niobium pentachloride is reacted in a fused salt bath with an alkali metal luoride to form an alkali metal niobium fluoride, the improvement which comprises reacting alkali metal niobium uoride in the fused salt bath with zinc in elemental form whereby a niobium-zinc reduction product is produced as a solid precipitate, and recovering niobium from said niobium-zinc reduction product.

2. The method according to claim 1 in which the fused salt bath is maintained at a temperature Within the range of from about 700 C. to about 880 C.

3. The method according to claim 1 in which the zinc is provided below the salt bath as a molten pool.

4. lThe method according to claim 1 in which zinc is provided below the fused -salt bath as a molten pool and an inert gas is fed into the molten pool of zinc.

5. The method according to claim 1 in which Ithe zinc is .provided in excess `of the amount required to produce a reduction product consisting essentially of niobium and zinc inthe molecular ratio of 1 part niobium to 3 parts zinc.

6. In a method for the production of niobium from niobium pentachloride in which niobium pentachloride is reacted in a fused salt bath with an alkali metal fluoride to form -an alkali metal niobium fluoride, the improvement which comprises reacting alkali metal niobium fluoride in the `fused salt bath with zinc in ele- Ymentariorm to produc@ a niobiumrzinarsdtion Pwd-Y uct,y lseparating niobium-zinc reduction product from the fused -salt bath, separating excess zinc from the niobiummetal fluoride in amount suicient to combine with the niobiurn content of the niobium pentachloride to form alkali metal niobium fluoride ,double salt, maintaining a non-oxidizing atmosphere in the reaction zone, providing zinc in said reaction Zone exposed to and for reaction with the potassium niobium fluoride double salt to form a niobium-zinc reduction product, separating niobiumzinc reduction product Af1 0mgthe fused salt bath, and separating and Aseparately :recovering niobium and ,zinc from the niobium-zinc 4reduction product.

8. The method of producing niobiurn which comprises the steps lof injecting niobium peutachloride into a fused sodium chloride-potassium chloride salt mixture ycontained'in a reaction Zone which contains also an alkali metal iluoride in amount suicient to combine with the niobium content of the niobium pentachloride to -form alkali metal niobium uoride double salt, maintaining a non-oxidizing atmosphere and a temperature of from about 700 C. to about 880 C. in the reaction zone, providing Vz'iiicV V4in said 'reactionn ZoneV exposedV to and for reaction with the potassium niobium fluoride double salt to form a niobium-zinc reduction product, separating niobium-Zinc reduction `product from the fused vsalt bath,

and separating and separately recovering niobium and zinc from the niobium-zinc reduction product.

References Cited in the le of this patent FOREIGN PATENTS 

1. IN A METHOD FOR THE PRODUCTION OF NIOBIUM FROM NIOBIUM PENTACHLORIDE IN WHICH NIOBIUM PENTACHLORIDE IS REACTED IN A FUSED SALT BATH WITH AN ALKALI METAL FLUORIDE TO FORM AN ALKALI METAL NIOBIUM FLUORIDE, THE IMPROVEMENT WHICH COMPRISES REACTNG ALKALI METAL NIOBIUM 